This invention relates to a thermosetting resinous composition of the novel curing system which is suitable for the production of a paint. More specifically, the invention relates to a thermosetting resinous coating composition which comprises an acrylic copolymer containing as a functional group a hydroxyl group and a novel curing agent selected from N-acylpolylactam compounds, N-acylpolyimide compounds and mixtures thereof. This thermosetting resinous composition may be formed into either a powder paint or a solvent type paint.
In general, resinous compositions to be used for manufacture of powder paints are required to have the following properties:
(1) The composition should be solid at room temperature so that it can easily be pulverized to powder.
(2) The composition in the powdery state should not cause blocking at a temperature lower than 50.degree. C. and it should have a free flowability at such temperature.
(3) When baking is conducted at a temperature higher than 130.degree. C., it should exhibit melt flow characteristics suitable for obtaining a smooth surface coating and a suitable gelation time.
(4) At a casting temperature approximating 100.degree. C., premature cross-linking reaction should not be caused and the resin component can easily be kneaded with a pigment, a flow modifier, an antistatic agent and other additives.
(5) The composition should have, after baking, cross-linking density and glass transition temperature enough to provide a sufficient mechanical strength.
It is apparent that selection of not only a resin to be used as the substrate but also a curing agent (cross-linking agent) is very important for obtaining a resinous composition satisfying all of these requirements.
Epoxy resins have heretofore been used as resins for powder paints, but powder paints including an epoxy resin have a very low weather resistance when used in fields where the resulting coatings undergo outdoor exposure. As a resin providing a coating excellent in the weather resistance, there is known an acrylic resin including as a cross-linking agent a melamine derivative modified with an ether having 1 to 4 carbon atoms, but this resinous composition is defective in that blocking is caused with increase of the ambient temperature in summer and the storage stability is insufficient.
Further, a carboxyl group-containing acrylic resin and a polyester resin including as a cross-linking agent an epoxy resin such as triglycidyl isocyanurate are known as resins for powder paints. Also these resins are defective in that blocking is caused with increase of the ambient temperature and the storage stability is insufficient. Further, properties of coatings formed from these resins are inferior.
Still further, there is known a powder paint comprising a glycidyl group-containing acrylic resin and a polybasic carboxylic acid such as 1,10-decane-dicarboxylic acid. This composition, however, is insufficient in the pigment-dispersing property.
As commercial solvent-type paints to be baked, there have heretofore been used melamine-alkyd resin paints, melamine-acrylic resin paints, phenol-epoxy resin paints and the like. Namely, in most of solvent-type paints heretofore used in the art, a formalin type resin such as a melamine resin and a phenolic resin is used as the cross-linking agent. The formalin type resin is excellent in weather resistance, chemical resistance and water resistance, but it is insufficient in flexibility and adhesion. Further, free formaldehyde used for the production of such resin is left in the paint, and at the step of heating and baking, free formaldehyde is generated and released with advance of the curing reaction. Accordingly, the atmosphere in a painting workhouse is contaminated with formaldehyde and its smell irritates workers. As a curing agent or cross-linking agent overcoming this disadvantage, use of a blocked isocyanate or the like has been proposed, but satisfactory results cannot be obtained even by this proposal.